The invention relates to thermal radiation detection apparatus comprising at least one pyroelectric detector device for receiving radiation from a scene and generating a voltage signal at its output which varies in accordance therewith, the detector device output being coupled to an output of the apparatus via a signal processing circuit for processing the voltage signal generated by the detector device.
Thermal detectors using pyroelectric devices are generally well known. They are used for a variety of different purposes such as, for example, thermal imaging, infra-red line scanning, and intruder detecting. Although a single pyroelectric device comprising a single active element may be used in a simple system for example for detecting intruders, arrays of elements, arranged linearly or two dimensionally, are now commonly used in more sophisticated systems such as imaging. A pyroelectric element generally consists of a layer of pyroelectric material sandwiched between two electrodes. When the temperature of the pyroelectric material changes, for example as a result of the incidence on the element of infra-red radiation from a scene being viewed, electrical charges are generated at the electrodes. If the element is arranged as a capacitor in a suitable amplifying circuit the resultant current or the voltage developed across a resistor can be detected. Since the pyroelectric charge is generated only when the temperature of the element is changing it is necessary for the temperature to be varied continuously to obtain a continuous electrical signal. In a simpler system, the required change in temperature may be caused by movement within the scene being viewed, for example an intruder moving across a surveillance region, so that a detector output signifying the presence of a moving object is produced which can then be used to trigger an alarm. Alternatively, when an image of the scene is required it is possible to scan the scene being viewed over the detector using either a suitable optical system or by moving the detector so that relative motion between the detector and the scene is obtained.
Although these methods provide time modulation of the incoming radiation enabling the pyroelectric element or elements to respond, it has been considered preferable to employ an optical chopper in front of the detector. The optical chopper, which may be in the form of a rotating apertured disc, periodically interrupts the incident radiation, the pyroelectric element being exposed to a substantially constant reference temperature while the radiation from the scene being viewed is cut off. The frequency of interruption may be in the range of a few Hertz to several hundred Hertz.
Exposure of the pyroelectric element to a scene results in the temperature of the element, and the voltage generated, gradually increasing at a rate determined by the thermal time constant of the element, the thermal time constant being given by the ratio of the thermal capacitance to conductance. Constant exposure of the pyroelectric element to a static scene over a prolonged period will produce a transient signal which decays due to charge leakage effects in the detector device. Heat loss from the element is radiative and is also affected by the heat sink effect of a substrate on which the element is mounted. When thermal equilibrium is reached, no signal is produced. Thus when a chopper is used, signal output is dependent on the chopping frequency, which in turn is chosen having regard to the thermal and electrical time constants of the detector device. Assuming the detector has a comparatively long thermal time constant, chopping of the incoming radiation results in the temperature curve of the pyroelectric element having an a.c. component superimposed on a gradually increasing d.c. component which reaches a plateau when thermal equilibrium is attained. The a.c. component produces a generally corresponding a.c. output voltage which is substantially proportional to the incoming radiation. Heretofore, use has been made in thermal detectors using choppers of a signal differencing processing technique in order to enhance the quality of the output signals. Differencing processing has been used with pyroelectric imaging sytems, for example with pyroelectric vidicons, for many years now and provides beneficial processing functions. An example of this kind of processing, applied to an imaging sytem, is described in U.S. Pat. No. 4,072,863 to which reference is invited. Briefly, the output signals from each of a number of pyroelectric elements in any array are gated in turn by a respective FET under the control of a shift register to a memory or delay line and also to one input of a differentiating amplifier. The memory output is connected to the other input of this amplifier. Signals produced by each respective element in successive frame periods corresponding to the periods in which the element is alternatively exposed to a scene and shielded by the chopper are thus supplied to the differentiating amplifier and the memory. The memory delays the passage of these signals to the amplifier by one frame period so that the amplifier produces a signal representative of the differences between a pair of successive signals from the element produced by the chopper open and closed states respectively. Because during the chopper closed states the element is allowed to cool, for example by radiation, the element signals will be of opposite polarity to those produced during the chopper open states. Hence the amplifier effectively adds the signals produced in successive chopper frames. Fixed pattern noise, which is substantially constant for open and closed frames, is therefore cancelled out by the differential amplifier. The output from the differential amplifier is fed via a switch synchronized with the chopper alternately to the inverting and non-inverting inputs of an inverting amplifier to produce a sequence or train of signals of the same polarity. Each of these signals is then fed to a summing circuit where it is added to the signal stored in a memory. The summed signal is then fed back into the memory. Each time a signal in the sequence is applied to the summing circuit it is added to the stored signal in the memory. In this way the signals produced by each pyroelectric element are integrated over a predetermined period of time. The output from the memory is fed to a utilisation device such as a display.
A processing technique having similarities is described in the article entitled "The Design of Low-Noise Arrays of MOSFETs for Pyroelectric Array Readout" by Watton and Manning published in SPIE Vol. 807, Passive Infrared Systems and Technology, pages 98 to 105, 1987 to which reference is also invited. Again, a differencing processing technique is described in which output signals from an individual element in an array from two consecutive field periods during which the chopper is open and closed respectively, and separated accordingly by a field time assumed to be less than the thermal time constant, are subtracted using a field delay. The output from this processor is stripped of the offset voltages which vary from element to element and are responsible for fixed pattern noise. This processing technique acts, therefore, as a filter for low frequency noise. In a detector in which comparatively long thermal time constants are present a modified image difference processing technique involving three consecutive field signals and requiring a frame (equal to two fields) store may be used.
Reliance is placed on the use of a chopper in these known apparatus for satisfactory operation. Although it is feasible for a detection apparatus to be operated without a chopper, in this case it is possible to obtain an output signal which varies with changes in received radiation using simple image differencing techniques. However, the output from the apparatus will not represent the changes in the received radiation faithfully. This is due to the effects of the finite thermal and electrical time constants involved. Without a chopper the shape of the output signal would contain faults, or anomolies.
Whilst a chopper has therefore, been considered important to satisfactory performance of a pyroelectric detector, its use is not without attendant disadvantages. The chopper, usually an electromechanical component, adds to the bulk and weight of the equipment and requires a suitable power supply. Moreover, when used with a thermal detector consisting of an array of pyroelectric elements there is a need to synchronise the action of the chopper with electronic scanning of the elements of the array. The use of a chopper also means that the amount of radiation received from a scene over a period of time is significantly reduced compared with that possible when an element is continuously exposed to the scene.